The invention relates to a pneumatic tire, and more particularly to a non-pneumatic tire made of reinforced elastomer. The invention is directed to a novel reinforcement element, for example, as a substitute for steel cords or aramid cables which are used for reinforcing tires.
An element of choice, which is widely used for reinforcing pneumatic tires, is the steel cord. It is known that the technique of cabling enables the reinforcement element to achieve relatively low radii of curvature while enabling it to withstand high stresses. A large number of elementary wires of small section are assembled such that, despite a cumulated section which is sufficiently large to achieve the desired resistance potential, each individual section remains sufficiently small to permit small radii of curvature without achieving permanent plastic deformation.
Using an assembly of elements of small section also makes it possible, in the case of steel, to limit the flexural rigidity. The flexural rigidity is the product of the Young's modulus and the moment of inertia of the section.
Many other materials are used, in particular textile materials. Mention may be made of rayon, nylon or, to mention a more modem material, aramid. However, the majority of uses do not make it possible to avoid having to assembly a plurality of filaments of small section in order to be able to achieve required performances in terms of transmission of forces and ability to be deformed. Unfortunately, the fact of needing to use an assembly, most frequently by plying in the case of a textile, restricts the properties of modulus in extension and does not impart, or imparts only little, flexural rigidity to the assembly. On the other hand, the microscopic size of the elementary filaments which form the textile spun yarns makes it possible for them to have relatively small radii of curvature. Although, in the belt of a radial tire, the textile plied yarns yield a saving in weight which is beneficial to certain aspects of the rolling resistance and eliminate the problems of corrosion, their lack of flexural rigidity, and in some cases of modulus in extension, does not make it possible to guarantee the excellent guiding stability and the wear resistance of the steel belt.
Instead of steel reinforcement elements, it has already been proposed, for example in patent application EP 0 475 745, to use a textile elongate composite element having essentially the following characteristics: the elongate element must be elliptical or rectangular; it comprises fibers selected from among aramid, glass, PVA and carbon; the initial modulus of extension of the impregnation resin used must not exceed 1.5 GPa. The selection criterion for these fibers which is proposed is a high tenacity (specific breaking load), greater than 15 g/denier (or 136 g/tex). However, aramid, PVA and partially carbon, unlike glass, have an acknowledged intrinsic weakness in terms of compressive strength. This disadvantage becomes apparent in particular in the application of these fibers to reinforcing tires, and results from their acknowledged weakness in terms of compressive strength. Doubtless in order to attempt to overcome this disadvantage, it is proposed to associate these fibers with a resin of low rigidity, which, for a given curvature imposed on the elongate composite element, stresses said element less. However, this choice causes some problems because it is not possible sufficiently to guarantee, over the entire life of the tire, sufficient compressive strength for belt reinforcement elements, which are under great bending stress, particularly at the edges of the triangulation plies in the belt.
To increase the flexural rigidity, polymeric textile products of a high Young's modulus in the form of monofilaments may be used, for example, aramid monofilaments of a diameter of the order of one tenth of a millimeter or several tenths of a millimeter, and these may be cabled in the manner of steel wires. By way of illustration, we shall mention Patent WO 92/12018. However, the very low intrinsic critical compression threshold of this type of product, which is defined as the maximum deformation in compression before the structure collapses, makes the assembly very fragile with respect to compressive stresses. There may result rapid, irreversible degradation in compression of the assemblies, hence the great difficulty of using anything other than steel for triangulation plies in the belt of the tires, because the drift of a tire causes bending on edge of the belt which is located beneath the tread, which stresses in compression certain parts of the reinforcement elements.
Another way of using textile fibers of high modulus and high tenacity (aramid fibers, aromatic polyester fibers—for example Vectran—, polybenzobisoxazole) consists in producing a monodirectional elongate composite used without cabling or equivalent operation. Depending on the volume content of the reinforcement element, it is possible to obtain a Young's modulus greater than that of a textile plied yarn. The bending modulus is very close to the modulus of extension and there is a real flexural rigidity, which can be modulated according to the choice of size and form of the section. However, such products have an intrinsic weakness in compression, that is to say a low breaking stress in compression linked with the use of textile fibers which themselves have a low, or even very low, critical threshold of deformation in compression. Now, it is known that use as a belt reinforcement for radial tires requires sufficient ability of the reinforcement element to resist compression.